How to make sqare/sharp lines with Grease pencil?
If I understand the question, you want tapered straight lines?
When I tried this with default settings I got faded ends. But I found that this can be fixed if you:

Turn Off Strength Pressure (which should be all you need to do)
Adjust the Thickness Profile to move the outer Left &amp; Right control points slightly up from the bottom of the graph (which is least thickness)